Atomic clocks are very accurate and stable time keeping devices that use the natural vibrations of an atomic system for regulation. Atomic clock accuracy is increasingly desirable in portable navigational systems for improved positional accuracy. Miniaturized versions of the atomic clock, sometimes referred to as a chip scale atomic clock (CSAC), use integrated circuits built with advanced semiconductor processes. The stability of the operating environment is a challenge for CSAC devices. An atomic clock can be divided into three parts: the physical portion (or physics cell); the electronics portion; and the package. The three pieces may be fabricated separately and assembled later.
Physics cells of CSAC's are sometimes built using MEMS technology. A typical physics cell has at least a laser source, some passive optical components (lenses, quarter wave-polarizer, etc.), a chamber including an atomic gas, a photodetector, a heater, a thermal sensor, and a field coil to induce an internal magnetic field through the atomic chamber. The electronics portion of the CSAC includes control electronics and a voltage to be applied to a voltage controlled oscillator (VCO) to produce an output clock. A ceramic module typically forms a package containing the physics cell and electronics.
In an atomic clock the laser excites the external electron of the alkali atoms of the cell (typically Cesium or Rubidium) from the ground state to an excited state. These quantum transitions are affected by the Zeeman Effect that splits degenerate transitions at zero magnetic field into a number of different energy states at a finite magnetic field. The primary frequency being somewhat insensitive to the magnetic field (order zero) and the higher order resonant frequencies having more and more sensitivity to the magnetic field. With a magnetic field in place, the quantum transition frequencies spread apart, and regulation circuitry in the electronics can distinguish and lock to these quantum transition frequencies.
In prior approach atomic clock designs, the resonant frequencies that are spread by the internal magnetic field are sensitive to changes in the external magnetic field. To prevent stray external magnetic fields from adversely affecting the frequency response of the system, a magnetic shield, or sometimes several magnetic shields, are placed around the atomic chamber, the physics cell and sometimes around the entire CSAC. Magnetic shielding is bulky, difficult to design and expensive. In a stationary atomic clock, the external magnetic fields can be calibrated out once the device is mounted in place. In portable atomic clock devices such as are increasingly used, fluctuations in the external magnetic fields will occur, and improvements are therefore needed.